Origin of Granite Domes in the Southeastern Piedmont
Author(s): William A. White
Source: The Journal of Geology, Vol. 53, No. 4 (Jul., 1945), pp. 276-282
Published by: The University of Chicago Press
Stable URL: http://www.jstor.org/stable/30061952
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ORIGIN OF GRANITE DOMES IN THE SOUTHEASTERN PIEDMONT
WILLIAM A. WHITE
University of North Carolina, Chapel Hill
ABSTRACT
The forms of the granite domes of the Southeast, which heretofore have been attributed to exfoliation,
are regarded as the product of granular disintegration brought about by chemical weathering.
INTRODUCTION
There are many dome-shaped exposures of granite throughout the Southeastern Piedmont. The largest, the wellknown Stone Mountain of De Kalb
County, Georgia, is well known; but its
many smaller counterparts throughout
the Piedmont of Georgia and the Carolinas are very little known. The domes
vary greatly in size from the massive
Stone Mountain itself, which is about a
mile and a half long and some 650 feet
higher than the surrounding peneplain,
down to small inconspicuous bosses,
little larger than residual boulders. They
are all alike, however, in having smooth,
spheroidal surfaces without sharp protuberances or re-entrants. Most are composed of granite, but some few are gneissic. None show visible joint systems.
Heretofore they have been explained
as the product of exfoliation; but the
writer, after several years of field work
throughout the region in which they occur, has come to believe that they have
resulted largely from the action of other
processes. The validity of the exfoliation
theory of origin for granite domes in certain other climatic regions is not questioned, but those which have developed
from the granites and gneisses of the
southeast do not show enough evidence
of exfoliation to justify the assumption
that it has played a dominant or even an
important part in their formation. It is
true that some of the southeastern domes
show a little exfoliation; but in most instances the surface is smooth and unbroken, and evidence of exfoliation is
rarely seen (Fig. i).
The exfoliation theory of dome development stands upon the essentially
sound reasoning that an unjointed mass
of homogeneous rock will be attacked
most readily on those parts of its surface
which have the smallest radii of curvature and that the continuation of such
selective attack will reduce the mass to a
spheroidal form. But this principle applies equally well to the denudation
caused by any other weathering agent
which is nondirectional in its attack. In
the southeastern states the climate is
warm and humid, and the remarkable
development of chemical weathering has
long been a matter of comment. Hydration, oxidation, and carbonation are
nondirectional in their attack and should
be adequate to reduce an irregularshaped mass to spheroidal form provided
the rock resistance is uniform. Since the
granites and gneisses from which the
domes have been sculptured are quite
homogeneous and without joints, there
is little reason to believe that their resistance to chemical weathering should
be differential.
While chemical weathering has usual276
GRANITE DOMES IN THE SOUTHEASTERN PIEDMONT
ly been most pronounced in areas underlain by jointed or schistose rocks, there
is good evidence that it has also had a
significant effect upon the unjointed
masses of granite and gneiss from which
these domes have been shaped. Quarrymen working .on them usually find it
necessary to remove 6 inches to several
feet of sap before they encounter unaltered rock. These are not impressive
thicknesses, but kaolinization character-
FIG.i.-Stone
Carolina.
Mountain in Wilkes County, North
istically takes place on the intergranular
surfaces of the feldspars, and the rock at
the surface of the exposure tends to break
up into a gruss which is washed off the
steep bare slopes as quickly as it is
formed. Many observers have noted this
intergranular alteration. L. E. Smith'
has made petrographic studies showing
it in the surficial phases of the unjointed
granite masses of the South Carolina
Piedmont, and the writer has made
similar studies in North Carolina.
EVIDENCE
FOR EXFOLIATION
It is evident that exfoliation and granular disintegration are coexistent as
' "Weather Pits in Granite of the Southern Piedmont," Jour. Geomorph.,Vol. II (1941), P. 125.
277
sculpturing agents on the domes, and
their relative importance should be determined by a comparison of the evidence for each. Considering first the case
for exfoliation, if a dome were dominantly the result of its action, one would expect to find that fact manifest in two
ways. (i) Since few exfoliation spalls detach themselves from the parent-mass
in the form of complete lenses, one would
expect to find the surface of the dome
covered by truncated remnants of spalls
which had partially fallen away. There
would probably be overlapping of such
remnants, and the surface of the dome
would have a somewhat imbricate appearance. Half Dome in Yosemite (Fig.
2) offers an excellent example of such a
surface. (2) If the surface of the dome
had been produced by exfoliation, at its
base one would expect to find a talus
slope composed of fallen and broken
spalls.
In the case of the domes of the Southeast neither of these criteria is satisfied.
As stated above, there is very little evidence of exfoliation on the dome surfaces. Looking at the face of Stone Mountain in Wilkes County, North Carolina,
the observer can see the broken edges of
no more than two or three spalls from
any one viewpoint. And at the base of
the mountain only a few remnants of
fallen spalls can be found. This observation applies equally well to the attendant
bosses and minor domes which appear
near by. On Stone Mountain in De Kalb
County, Georgia, there is somewhat better evidence of exfoliation; and it seems
to have been a slightly more important
factor in the denudation of that mass.
Even there, however, the broken edges
of spalls are so rounded by normal weathering (granular disintegration) that they
are not conspicuous. Other less wellknown domes throughout the region
278
WILLIAM A. WHITE
show the effects of exfoliation in varying
intensity, but most of them are as little
affected by it as is Stone Mountain in
Wilkes County, North Carolina. Some
are affected even less.
It is true that there seems to be a latent tendency toward hypogene exfoliation in all the domes, but there does not
appear to be much evidence that it has
mote, for there is no evidence of an arid
climate in this region later than Triassic
time.
In brief, if a dome were sculptured by
exfoliation, the evidence for the action
of that agency should be spectacularly
displayed as remnants of broken spalls
both adhering to the dome surface and
composing a talus slope at its base.
FIG. 2.-Half Dome in Yosemite Valley, CaliforniA after F. E. Matthes. Photograph by courtesy of
the U.S. Geological Survey.
ever been activated by wholly natural
agencies. In every place where it can be
recognized, it has been initiated artificially by the rapid removal of overburden
in the process of quarrying. Most of the
natural spalls are quite thin and seem to
be the result of other causes.
It is possible that the domes were produced by exfoliation under different climatic conditions in a previous geologic
age. However, this possibility seems re-
EVIDENCE FOR GRANULAR
DISINTEGRATION
On the other hand, if a dome had developed through granular disintegration,
the evidence to prove it should be somewhat obscure. Formation of gruss is a
grain-by-grain process. As soon as a grain
has been loosened from the parent-mass,
it is washed off the steeply sloping surface either to become part of the bed
load of the small drainage ways at the
GRANITE
DOMES
IN THE SOUTHEASTERN
base of the dome or to be incorporated
in alluvial fans surrounding it. Because
of the slow rate at which the individual
grains are released from the parent-mass,
there is small tendency for fans to develop; but in every location which is favor-
FIG. 3.-Map
soil series.
PIEDMONT
279
Field evidence2 suggests that it develops
largely upon unjointed granites and
gneisses as the product of granular disintegration. The intimacy of its association with the domes is exemplified by the
soils map shown in Figure 3. This has
of area around Stone Mountain in De Kalb County, Georgia, showing distribution of
able to the detention of sediment there
are deposits of the coarse debris produced
by granular disintegration. Furthermore,
every dome of the writer's acquaintance
is surrounded for significant distances by
the young soils which result from granular disintegration. The dominant series
is the Louisburg, a light-colored sandy
soil without definite profile development.
been reproduced from unit-area maps of
the Soil Conservation Service, United
States Department of Agriculture,3 and
2 W. A. White, "Determining Factors in the
Coloration of Granite Soils in the Southeastern
Piedmont," Amer. Jour. Sci., Vol. CCXLII, No. 7
(1944), PP. 361-63.
3 P. H. Montgomery, "Erosion and Related
Land Use Conditions of the Lloyd Shoals Reservoir
Watershed, Georgia," Phys. Surv. Div., Soil Cons.
280
WILLIAM A. WHITE
shows the area surrounding Stone Mountain in De Kalb County, Georgia. It will
be noted that the dome is surrounded by
the young Louisburg series, although the
region in general, away from the dome,
is overlain by the more common Cecila mature series which, as the writer has
shown elsewhere,4 characteristically develops on closely jointed acid rocks. The
area covered by Louisburg extends for
some distance to the east of Stone Mountain, where it surrounds a number of
smaller low domes or "flat rocks." Two
of these are shown on the map, to the
northeast of Stone Mountain.
On sheet 64 of the same series of maps
from which Figure 3 was reproduced a
similar distribution of Louisburg soil
may be seen surrounding Pine Mountain, which is located about I mile east
of Lithonia, Georgia. This is a smaller
dome than Stone Mountain and has been
reduced to a lower profile, but it also
gives every manifestation of having been
produced by granular disintegration
rather than exfoliation. Many other
small domes in all stages of reduction appear throughout the area east of Stone
Mountain, Georgia. All of them are surrounded by Louisburg soil and show
little evidence of natural exfoliation.
VALLEY-WALL FLAT ROCKS
In general, the upland surface of the
Piedmont is a peneplain which is very
poorly developed along its western edge
but shows increasing development to the
east. At its easternmost limit, near the
edge of the Coastal Plain, it has been reduced to a very low relief, and monadnocks are rare or absent. There has been
uplift and some dissection. The domes
Serv., U.S. Dept. Agric., 1940. Parts of sheets 33,
34, 46, and 47.
4 White, pp. 361-63 of ftn. 2 (1944).
differ greatly in topographic age, as
measured by their local relief. In the upper Piedmont, where peneplanation is not
as well developed, they are largely mature with high profiles and steep sides.
In the middle Piedmont they are in general older and of lower relief, many being
almost flat. Near the Coastal Plain,
where peneplanation was most highly developed, their former presence is represented by exposures along valley walls
where dissection is taking place.
These last are characteristically wide,
slightly inclined, and broadly curved
exposures of unjointed granite with
surfaces unbroken save for occasional
weather pits. Uphill they pass under
residual soils of the Louisburg or Durham series, which in some places are
capped by a veneer of marine deposits.
Typical examples are Flat Rock and
Forty-Acre Rock near Kershaw, South
Carolina.
From their topographic positions in
the walls of young valleys and from the
fact that they pass under residual soil,
one draws the conclusion that the present
exposures of these flat rocks were sculptured by a later erosion cycle than that
which shaped the monadnock domes
higher in the Piedmont. However, since
these areas were highly peneplaned, it
would seem plausible to believe that
these valley-wall flat rocks merely represent new exposures of former domes
which were reduced by peneplanation to
the point where they could develop a
residual soil. Geomorphically, such former domes would have been of the same
generation as the present examples found
in the upper Piedmont, although chronologically older; that is, because the peneplain developed first in the lower Piedmont and extended progressively westward, the former monadnock domes
which developed on the lower edge of it
GRANITE DOMES IN THE SOUTHEASTERN PIEDMONT
had been reduced to the general level and
covered by residual soil before the present dissection again exposed their flanks
as valley-wall flat rocks. On the other
hand, the present monadnock domes of
the upper Piedmont developed later with
the western migration of peneplanation,
and they have not yet been reduced to
the general level.
A continuation of this reasoning
would suggest that many of the areas of
Louisburg and Durham soil which are
found in undissected parts of the Piedmont also indicate the location of former
domes which have been reduced to flatrock status and buried by residual soil.
In those instances where valley-wall
flat rocks pass under remnants of Tuscaloosa or "Lafayette" deposits, it is, of
course, possible that former low domes
were planated by wave erosion. However, the absence of any wave-cut scarp
at the edge of the Coastal Plain argues
against this possibility and suggests that
peneplanation had been extreme before
the last submergence.
Where valley-wall flat rocks pass under marine terrace deposits, it is obvious
that the localization of dissection has
been the result of superimposition from
consequent drainage. Therefore, the development of spheroidal surfaces on the
granites exposed in the walls of such
narrow valleys demonstrates that domes
can develop independently of any control which might be exerted by internal
structures of the granite itself, such as
concentric strain lines, schlieren, or petrofabric orientation. In several such
places, as at Forty-Acre Rock near Kershaw, South Carolina, spheroidal surfaces appear on both sides of the dissecting valley-a fact which argues further
against the influence of internal structures, for the directions of curvature are
reversed on opposite sides of the valley.
INFLUENCE
OF INDURATED
281
VENEERS
Another factor which is believed to
have had some significance in giving the
domes their smooth spheroidal surfaces
may be found in the indurated veneers
which characteristically appear on their
exposed surfaces.5 These veneers seem to
result from the deposition and oxidation
of iron compounds which have been carried upward in solution by capillary
water. Deposition takes place near the
surface when the water evaporates and
serves to reconsolidate the partially disaggregated mineral grains of the altered
sap rock. Such indurated veneers on the
domes tend to slow weathering and
topographic reduction both by restricting the entrance of air and surface water
to the underlying rock and by increasing
the resistance of the surface to mechanical disintegration. However, they are of
most influence in the development of
topographic form when they appear on
flat-rock exposures in areas subject to
dissection. There, by their tendency to
reduce weathering, they increase the differential in rate of decomposition between the exposed rock and that underlying the surrounding soil.
When dissection takes place, this
sharply marked distinction between altered and unaltered rock at the edge of
the old exposure localizes the steep edge
of a table rock of the type shown in Figure 4, in which a veneer can be seen preserving the surface of a former small flat
rock. After prolonged exposure such a
form will lose the sharp protective edges
of its original veneer; but, as the edges
are rounded, it will develop an extension
of the veneer down its sides and resolve
at length into a small dome. This new exs White, "Geomorphic Effects of Indurated
Veneers on Granites in the Southeastern States,"
Jour. Geol., Vol. LII (1944), pp. 333-41-
282
WILLIAM A. WHITE
tension to the original veneer will have
similar protective qualities and will tend
to preserve the sharp boundary between
the dome and the surrounding soil. This
suggest that perhaps geologists have
erroneously considered granite domes to
be unusual land forms, produced only by
very special conditions. It would prob-
FIG. 4.-Granite outcrop protected by indurated veneer, 2 miles southwest of Wendell in eastern Wake
County, North Carolina.
process may possibly explain the sharp
knick points which characteristically appear at the bases of domes, and (although
the writer does not like to extend his conclusions to regions unfamiliar to him) it
may have some connection with the
genesis of the bornhardts of East Africa.
In conclusion, the writer would like to
ably be more catholic to regard them as
the expected form wherever nonjointed
homogeneous rocks are subjected to the
attack of any nondirectional agency of
denudation. The particular agency would
be determined by the local climate, but
the resulting land forms should be essentially similar.